The mass number of an atom is equal to the total number of protons plus neutrons in the atom.

Atoms with identical atomic numbers but different mass numbers are called isotopes. Isotopes have the same number of protons but different numbers of neutrons.

Isotopes are represented by symbols like these: The symbols represent "carbon twelve", carbon-12 and "carbon fourteen", carbon-14. The mass number is on the top, the atomic number is on the bottom.

Its all about stability!

While many isotopes of an element are possible, some neutron/proton ratios are much more likely than others. In general, the larger the atom (proton number), the greater the number of neutrons required to maintain stability. See a graph of the "Island of Stability".

Most elements occur in nature as mixtures of isotopes.

The average atomic mass, also called atomic weight, of an element is determined by using the masses of its various isotopes and their relative abundances.

Calculating Average Atomic Mass:

Natural carbon is composed of 98.93% carbon-12 (exactly 12 u) and 1.07% carbon-13 (13.00335 u).

(0.9893)(12 u) + (0.0107)(13.00335 u) = 12.01 u